Naphtha produced from various refinery units, including the Crude and Vacuum Unit, Coker Unit, and others, forms a significant part of the gasoline product pool, and is a major source of the sulfur found in gasoline. This sulfur impurity typically must be removed to comply with various product specifications and/or environmental regulations.
Hydrodesulfurization is a catalytic chemical process used to remove sulfur from a naphtha feed, as well as other petroleum products. Removing the sulfur from these products reduces sulfur dioxide emissions that result from use of the products in fuel combustion. Moreover, removing the sulfur provides benefits within a petroleum refinery because sulfur, even in very small quantities, can adversely affect noble metal catalysts used in downstream processes, such as catalytic reforming.
Conventional fixed bed hydrodesulfurization can reduce the sulfur level of a naphtha feed. During hydrodesulfurization, the raw feed reacts with hydrogen in the presence of a catalyst to convert organic sulfur compounds to hydrogen sulfide. However, the hydrogen sulfide formed during the hydrodesulfurization recombines with olefins in the naphtha feed to form mercaptans, known as recombination mercaptans. In particular, it has been found that presence of recombination mercaptans is increased when the operating temperature of the hydrotreating reactor is high. For instance, temperatures in excess of 650° F. have been found to increase formation of recombination mercaptans.
The hydrodesulfurization process generally involves a hydrogenation reaction, which cleaves a chemical bond between a carbon atom and a sulfur atom in the hydrocarbon. This hydrogenation reaction is exothermic, causing the temperature of effluent leaving the hydrotreater reactor to increase over time. This can cause a corresponding increase in the formation of recombination mercaptans. A post-treat reactor is required to remove the recombination mercaptans by converting mercaptans into hydrogen sulfide.
Accordingly, there is a need to control the temperature of the naphtha feed flowing through the post-treat reactor.